1. Field of the Invention
The invention relates to improvements in processes for shape selective preparation of aromatic hydrocarbon compounds. More specifically, the invention relates to improvements in processes for preparing nitrogen-containing aromatic hydrocarbons such as pyridines and picolines.
2. Description of the Prior Art
Pyridine is a six-membered heterocyclic aromatic compound with one nitrogen atom in the ring structure. It is an important chemical in the manufacture of agricultural chemicals, e.g., herbicides and pesticides, and pharmaceuticals, and is also useful as a solvent in the polymer and textile industries. Important derivatives of pyridine include, for example, nicotinic acid and nicotinamide (vitamins essential for human health), chlorpheniramine (an antihistamine), cetylpyridinium (a germicide and antiseptic), isoniazid (an important antitubercular drug), and Paraquat.RTM. (a herbicide).
Pyridine itself is a simple ring structure, and has only hydrogen atoms bonded to the structure. Pyridines having one methyl group attached to the ring structure are called methylpyridines or picolines, and they include 2- or .alpha.-picoline, 3- or .beta.-picoline, and 4- or .gamma.-picoline. Dimethylpyridines are called lutidines, and the 2,6- and 3,5-lutidines are readily obtainable. Trimethylpyridines are called collidines, with the 2,4,6-collidine or sym-collidine being the most common. See "Pyridine and Pyridine Derivatives," Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 19, 3rd. Ed. (1982).
Pyridine and picolines can be obtained as by-products of the coal tar industry or coke manufacture. However, pyridine is found in only small amounts in coal tar, and a preferred method of obtaining pyridine is by chemical synthesis. Chemical synthesis typically relies on a catalytic gaseous reaction (condensation) between ammonia (or amines) and carbonyl compounds such as aldehydes or ketones. However, these chemical synthesis methods have historically suffered from disadvantages of low yields and poor selectivity, and short operation cycle and catalyst lifetime.
The term "base synthesis" is known and used in the field of pyridine chemistry to identify synthetic processes by which bases of pyridine and its alkylated derivatives are prepared by reacting aldehydes and/or ketones with ammonia in the gas phase using a heterogeneous catalyst. For example, the reaction of acetaldehyde with ammonia in the presence of heterogenous catalysts at about 350.degree. C. to about 550.degree. C. yields 2- and 4-methylpyridines (.beta.- and .gamma.-picolines). As another example, acetaldehyde, formaldehyde, can be reacted with ammonia to yield pyridine and 3-methylpyridine. Such pyridine synthesis methods are described, for example, in U.S. Pat. No. 4,675,410 to Feitler and U.S. Pat. No. 4,220,783 to Chang et al. which are each herein incorporated by reference.
Reaction of acetaldehyde or certain other low molecular weight aldehydes and ammonia either in the absence or presence of methanol and/or formaldehyde to yield pyridine and alkyl derivatives thereof has been carried out in the presence of amorphous silica-alumina composites containing various promoters. See, for example, U.S. Pat. Nos. 2,807,618 and 3,946,020. The yields of desired products using the latter catalysts have been poor. Alkylpyridines have also been synthesized, as reported in Advances in Catalysis, 18:344 (1968), by passing gaseous acetaldehyde and ammonia over the crystalline aluminosilicates NaX and H-mordenite. While initial conversion utilizing these materials as catalysts was high, catalyst deactivation by coking was rapid, providing a commercially unattractive system, characterized by poor catalytic stability.
Amorphous aluminosilicate catalysts provide a reasonable yield of pyridine at the beginning of the process; however, after repeated operation cycles some limitations appear which make these catalysts unacceptable from a commercial standpoint.
Synthetic crystalline zeolites having an intermediate pore size as measured by the Constraint Index of the zeolite being between 1 and 12, e.g., ZSM-5, have been found to provide commercially useful yields and product selectivities. U.S. Pat. No. 4,220,783 was pioneer in this discovery, teaching synthesis of pyridine and alkylpyridines by reacting ammonia and a carbonyl reactant which is an aldehyde containing 2 to 4 carbon atoms, a ketone containing 3 to 5 carbon atoms or mixtures of said aldehydes and/or ketones under effective conditions in the presence of a catalyst comprising a crystalline aluminosilicate zeolite having been ion exchanged with cadmium and having a silica to alumina ratio of at least about 12, and a Constraint Index within the approximate range of 1 to 12.
Use of a ZSM-5 catalyst component in a fluidized or otherwise movable bed reactor is taught in U.S. Pat. No. 4,675,410. U.S. Pat. No. 4,886,179 teaches synthesis of pyridine by reaction of ammonia and a carbonyl compound, preferably with added hydrogen, over catalyst comprising a crystalline aluminosilicate zeolite which has been ion exchanged with a Group VIII metal of the Periodic Table. The crystalline aluminosilicate zeolite has a silica to alumina mole ratio of at least 15, preferably 30 to 200, a Constraint Index of from 4 to 12, e.g., ZSM-5, and the process provides a high and selective yield of pyridine.
U.S. Pat. No. 5,013,843 teaches addition of a third aldehyde or ketone to a binary mixture of aldehydes and/or ketones used in preparing mixtures of pyridine and alkyl-substituted pyridines in large scale continuous processes. In a preferred system, propionaldehyde is added to a binary mixture of acetaldehyde and formaldehyde to produce beta-pyridine and pyridine. The catalyst for this process is a crystalline aluminosilicate zeolite in the acidic form having a Constraint Index of from 1 to 12, e.g., ZSM-5.
U.S. Pat. No. 5,218,122 to Goe et al. relates to a process for base synthesis over a modified catalyst containing tungsten, zinc, or tin, and a constraint index of about 1 to 12.
These crystalline catalysts have been provided in an attempt to improve the performance of amorphous catalysts. However, the yield of pyridine, and selectivity operation life, and restorability of the catalyst after regeneration still have not been satisfactory. Specifically, the yield of pyridine is low, the ratio of pyridine to picolines is not suitable, and the yield of the desired pyridine product decreases sharply after several reaction/regeneration cycles.
In view of the above considerations, it is clear that existing processes for synthesis of pyridines, picolines, and related compounds is encumbered by disadvantages that render the processes unacceptable from a commercial perspective. Specifically, (1) large amounts of carbonaceous deposits form which reduce the activity of the catalyst to below acceptable levels; (2) the selectivity of the reaction for the desired pyridine and alkylpyridine becomes very poor; (3) the catalysts become thermally unstable; and (4) the activity of the used catalyst is difficult to restore completely by regeneration.
Accordingly, it is one of the purposes of this invention to overcome the above limitations in pyridine and picoline synthetic processes, by providing a process by which high yields and high purities of desirable products can be achieved in a cost- and time-efficient manner. It is a further purpose of this invention to provide a pyridine synthesis process that can be readily adapted and scaled to commercial application.